Optical fibers are powerful for many uses. In many applications, optical fibers are employed to cast illumination onto and/or gather light from a subject. In an important class of applications, the characteristics of light emitted on a subject under inspection are compared with the characteristics of light returning from the subject. To facilitate this type of application, one or more emitter fibers and one or more collector fibers may be arranged into an optical cable to create a fiber optic probe.
A fiber optic interface having desired optical or physical characteristics may be formed as an integral part of the optical probe, typically at the tip of the probe. Alternatively, the desired optical interface may be formed as a separate probe tip that may be attached to the end of an optical cable. To create the desired optical interface, the end faces of the probe fibers may be shaped to have desired optical or physical characteristics. In this manner, the probe may be designed to remotely analyze a subject under inspection to ascertain important parameters.
Oftentimes, the optical fibers must be removed from direct contact with the subject under inspection. The tip of the probe may be capped by a protective window, which results in stand off between the fiber end faces and the subject under inspection. Separating the fiber end faces from the subject under inspection while achieving acceptable optical performance has shown to be a major difficulty in fiber optic probes. Suitable fiber optic interface technology has not been available. The lack of devices meeting this requirement severely limits the utilization of many powerful analytical techniques, which would otherwise be suitable for remote analysis using optical fibers. Thousands of high-volume applications are pending the availability of suitable probes.
Avoiding direct contact between the probe and the subject under inspection is often required for several important reasons. First, physical isolation of the optical fibers is required in mechanically abusive environments. Second, isolation from chemical attack is required in chemically harsh environments. Third, stand off from very dark or opaque media is often required to facilitate concurrence of the illumination zone (i.e., the area illuminated by the probe emitter) and the reception zone (i.e., the field of view of the probe collector). Fourth, in many applications it is advantageous to orient the subject under inspection with respect to the fibers so that the effects of specific photon-matter interactions can be suitably isolated. The prior art shows numerous attempts to satisfy the need for improved fiber optic probes; however, it will be seen that each attempt is severely lacking.
To illustrate the shortcomings of prior art probes, FIG. 1 is a scaled, side cross-sectional view of the emission and reception patterns of a prior art probe tip 10 that is common in the industry. The probe tip 10 includes an optical bundle 12 having a flat end face profile positioned behind a disk-shaped window 14, which is typically formed of sapphire or diamond. An emitter fiber 16 is surrounded by a ring of equal-sized collection fibers 18, represented by the collection fibers 18a and 18b that are visible in the cross-sectional view. FIG. 1 illustrates two problems that are experienced with this type of probe: poor overlap between the illumination zone of the illumination fiber 16 and the reception zones of the collection fibers 18, and collection of back-reflections from the outer face 32 of the window 14.
The problem of poor overlap between the illumination zone 26 (shaded area) of the emitter fiber 16 and the reception zone 28 (cross-hatched area) of the collection fibers 18 results from the flat-faced interface profile, which causes the optical axis 22 of the emitter fiber 16 to be parallel with the optical axes of the collection fibers 18, as illustrated by the optical axis 24 of the collection fiber 18b. This causes poor coupling between the illumination zone 26 of the emitter fiber 16 and the reception zones 28 of the collection fibers 18. In fact, much of the reception zone 28 of the collection fiber 18b does not overlap with (i.e., is not illuminated by) the illumination zone of the illumination fiber 16.
The problem of collection of back-reflections from the outer face 32 of the window 14 is illustrated by the reflections 30a and 30b (closely-hatched regions) from the outer face 32 of the window 14. These reflections are cast directly into the collection fibers 18a and 18b without having interacted with the medium external to the window. This is an example of the more general problem of the collection of unwanted light, which is referred to as the collection of back-reflected, extraneous, or stray light. Unwanted light can propagate in the collection fibers 18 and, thus, drown out the desired light returning from the subject under inspection.
The problem of collecting of back-reflected light is exacerbated by the fact that extraneous light frequently propagates within the emitter fiber in conjunction with the principal source light. This extraneous light is often generated within the fiber by source photon interactions within the fiber core, such as silica-Raman and fluorescence. These phenomena may cause the fiber to become overfilled with the extraneous light. From the fiber's overfilled state, the extraneous light diverges from the fiber end face at a greater angle than that of normally-propagating light. In some optical interface configurations, this causes the back-reflected extraneous light to more readily enter the collection fibers.
Neither the emitted silica-Raman light nor its back reflection from the outer face 32 of the window 14 is depicted in FIG. 1. The emitted silica-Raman light diverges more rapidly from the end face of the emitter fiber 16 than the illumination zone 26 of the normally-propagating light illustrated in FIG. 1. Although the majority of the emitted silica-Raman light is located within the illumination zone 26 defined by normally-propagating light, a significant quantity of silica-Raman light is emitted from the end face of the emitter fiber 16 at angles beyond those at which normal propagation can be sustained. The portion of the emitted silica-Raman light that is outside the illumination zone 26 of the normally-propagating light may be back-reflected off the outer face 32 of the window 14. In the flat-faced interface profile of the probe shown in FIG. 1, this portion of the back-reflected silica-Raman light typically remains outside of the theoretical angular acceptance capabilities 28 of the collection fibers 18. Nevertheless, the effect of back-reflected silica-Raman light is an important consideration when designing probe interfaces that are not flat-faced.
Some attempts have been made in the prior art to address the problems described above. For example, McGee et al., U.S. Pat. No. 5,166,756, describes a probe for analyzing powders. In this probe, a multiplicity of illumination and collection fibers are arranged behind a sapphire rod that functions as a window. The outer face of the window is inclined relative to the end faces of the optical fibers. The inclined outer face of the window directs the back reflections from this surface outside of the reception capability of the collection fibers. This allows the subject to be analyzed with reduced interference caused by the back reflections from the end face of the window. In addition, the inclined outer face of the window may be used to mechanically orient certain materials to facilitate photonic analysis of a specific material parameter. For example, a planar surface of a powder grain may become aligned with the inclined outer face upon contact with the window, which may reduce the collection of specular reflections from the powder grain's planar surface and, thus, allow collection of a diffuse component of the reflected light.
The probes described by McGee et al. suffer from several drawbacks. First, the inclined outer face of the window separates the optical fibers from the sample by a significant distance, which reduces the coupling efficiency of the probe. Second, the overlap between the illumination zone of the emitter fibers and the reception zone of the collection fibers is not precisely controlled, which further reduces efficiency and requires the use of finely stranded fiber optic bundles. These finely stranded bundles are expensive and somewhat limited in their transmission characteristics. Third, the probe's optical characteristics are highly dependent on the position of the illumination and collection fibers relative to each other and relative to the window's outer face. Maintaining repeatability of these positional factors is difficult to achieve when fabricating probes. Therefore, probe-to-probe performance repeatability, particularly as it relates to broad band intensity, suffers.
McLachlan et al., U.S. Pat. No. 4,573,761, describes a probe in which the optical axis of an emitter fiber and the axes of multiple collection fibers are directed into intersection by bending the fibers near their ends. This method is difficult to incorporate into manufacture, produces variability, results in a restrictively large assembly, and causes losses at the sharp bends. From their positions behind a window, the collection fibers collect back reflections from the window that devastate measurement quality. Other methods described in the prior art include the use of lenses in the probe interface to direct an optical fiber's patterns of emission and reception. The introduction of these additional elements causes mounting, alignment, manufacturing, and sealing problems. As with the bends described by McLachlan et al., the introduction of additional elements into the probe interface decrease the robustness and increase the size of the probe interface.
FIG. 2 is a scaled, side cross-sectional view of another prior art probe tip 50 similar to those described in O'Rourke, et al., U.S. Pat. No. 5,402,508. The probe tip 50 includes a fiber optic bundle 52 positioned behind a sapphire window 54. The fiber optic bundle 52 includes a center emitter fiber 56 surrounded by collection fibers 58, represented by the collection fibers 58a and 58b visible in the cross-sectional view. The end faces of the collection fibers 58 are angled, which through refraction causes the optical axes 60a, 60b of the collection fibers 58a, 58b to converge towards the optical axis 62 of the emitter fiber 56. Typically, the angle between the end faces of the collection fibers 58 and the inner face of the window 54 is about 20.degree..
Although the probe tip 50 demonstrates certain improvements over the McGee et al. probe tip, it suffers from severe stray light collection that impairs measurement quality. That is, the reflections 64a and 64b from the outer face 66 of the window 54 are cast into the collection fibers 58a and 58b without having interacted with the medium external to the window. In fact, the stray light performance of the probe shown in FIG. 2 is worse than that of the flat-faced probe depicted in FIG. 1. This is due, in part, to the over-filled nature of silica-Raman light emitted from the emitter fiber 56. As noted previously, the over-filled silica-Raman light is emitted from the end face of the emitter fiber 56 at angles beyond which normal propagation can be sustained. This causes a portion of the over-filled silica-Raman light to be reflected off the outer face 66 of the window 54 at an orientation beyond the limits for normally-propagating light. Because the collection fibers 58 of the FIG. 2 probe are angled with respect to outer face 66 of the window 54, they collect more over-filled silica-Raman light than the flat-faced fiber shown in FIG. 1.
O'Rourke, et al. describes increasing the space between the fiber bundle 52 and the inner face of the window 54, apparently to reduce the collection of back-reflections from the window. However, the additional space severely reduces the coupling efficiency of the probe, increases the size of the probe interface, and presents manufacturing difficulties.
O'Rourke, et al. also teaches that reflections from outer face 66 of the window 54 are not troublesome for deployment in liquids; however, the opposite is often the case. Windows fabricated from strong, chemically resistant materials such as sapphire and diamond have refractive indices much higher than most solutions. For example, sapphire's refractive index is approximately 1.77, whereas water's refractive index is about 1.33. This refractive index differential results in reflection from light exiting the window. The reflection's strength and potential for entrance into adjacent collection fibers increases as the angle of light incidence diverges from perpendicular. Lower refractive index window materials such as silica can be employed; however, they are typically much weaker physically and, thus, require a thick window. The increased window thickness results in increased collection of back-reflected light and decreased coupling efficiency.
In sum, the prior art does not describe probe interfaces, or methods for designing and manufacturing probe interfaces, to overcome the drawbacks described above. The proliferation of unsuccessful attempts in the prior art to create suitable fiber optic probes demonstrates the technical complexity of the problem. Despite intense activity and numerous designs, a strong need persists for improved fiber optic interfaces that can be employed in instrument probes and other applications. In particular, a need exists for fiber optic interfaces, methods for designing fiber optic interfaces, and machines for manufacturing fiber optic interfaces that can readily reject the collection of stray light while efficiently collecting desired light that has interacted with the subject media.